Peak detection in sediment-charcoal records: impacts of...
Transcript of Peak detection in sediment-charcoal records: impacts of...
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Peak detection in sediment-charcoal records: impacts of alternative data analysis methods on fire-history
interpretationsPhilip E. Higuera*, Daniel G. Gavin, Patrick J. Bartlein,
Douglas J. Hallett
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Type of siteMethod of
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Small hollows
Small lakes
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Forest soilsDating of trees
and soil
charcoal
Interval
sampling of
sediment cores
Continuous
sampling of
sediment cores
Ages of most recent fires over a site network
Charcoal stratigraphy in sedimentSmall lakes
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Years before present
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Development of a charcoal stratigraphy
There exists a serious need for increased automation and quality control!
12 min/sample
3 min/sample
15 min/sample
Total effort=30 min. per sample800 samples = 400 hours
Higuera et al., IJWF, in press
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The ideal charcoal stratigraphy (Rockslide Lake, BC)
Gavin et al., Frontiers of Ecology and the Environment, 2007
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Charcoal accumulation rate (CHAR)20-year interpolation
Charcoal concentration Sed. rate
A problematic charcoal stratigraphy (Eleanor Lake, BC)
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Gavin, unpublished data
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Analytical steps for peak identification
Higuera et al., IJWF, in press
Resampling to constant time intervalDepth (cm) Year
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Sed. rate
(1+(7/10)*1) / 15 = 0.113 cm yr-1
Conc.
counts in 1 cm3 subsamples
((3/10)*1+(12/13)*1) / 15 = 0.082 cm/yr
10*(8/15)+32*(7/15)= 20.26 cm-3
CHAR
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32*(3/15)+53*(12/15)= 48.8 cm-3 4.00
CHAR per sample 10*(1/8)=1.25
32*(1/10)=3.2
53*(1/13)=4.07
Detrend-and-threshold peak identification
Higuera et al., IJWF, in press
Common dendrochronological methodsMost-often used with charcoal records
No transform; Residuals
Transform; Residuals
No transform; Index
Transform; Index
sed. rate
charcoal influx
log-transformed charcoal influx
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Peak threshold determination: Gaussian mixture models
Residual CHAR (pieces cm-2 yr-1)
Rela
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Charcoal influx series represents a mixture of processes that should be apparent in the distributions of charcoal values.1) Noise = unrelated to fires: sampling effects, analytical error.2) Signal = following fires: episodic events.
Signal to noise index (SNI) measures the distinction of the noise and signal distributions. SNI can determine suitability for peak ID. SNI = σ2S/(σ2S+σ2N)
Contrast this method with: statistics-of-extremes methods 1) Generalized extreme value (GEV)distributions or Pareto distributions2) Peak-over-threshold (POT)methods
Fit using CLUSTER by Charles Bouman, Perdue University.
Peak threshold determination: Gaussian mixture models
Skewness is a good property when using Gaussian mixture models.
Peak threshold determination: Gaussian mixture modelsand a sensitivity test
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Residual charcoal accumulation rate particles•cm-2•yr-1
Extreme values
thresholds
Upper Squaw Lake, Oregon.Three distributions fit (showing lower two only)....separating two classes of peaks.
Ruppert Lake, Brooks Range, AK
Higuera et al., PLoS One, 2008
Local Thresholding: GMM fit to moving windows: threshold is therefore fit to
“local” variability.
1. Interpolation to 15-year intervals2. Background detrending: Difference from 500-year loess regression3. GMM fit in 500-year moving windows
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Higuera et al., IJWF, in press
Minimum-count test for significant differences between two counts
• X1 and X2 are counts of samples 1 and 2
• V1 and V2 are volumes of samples 1 and 2
• d is normally distributed
H0: X1 and X2 are drawn from a single Poisson distribution.
• Detre, K, and C White (1970) Comparison of 2 Poisson-Distributed Observations. Biometrics 26, 851
• Shiue, WK, and LJ Bain (1982) Experiment size and power comparisons for two- sample Poisson tests. Journal of Applied Statistics 31, 130-134.
What is the effect of the detrending method?
Higuera et al., IJWF, in press
Peak charcoal is independent of
background (additive process)
Peak charcoal depends on background (multiplicative process)
s = # peaks in period 1 / # peaks in period 2
p b
typo!
s = # peaks in period 1 / # peaks in period 2
Higuera et al., IJWF, in press Higuera et al., IJWF, in press
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Marlon et al. Nature Geosciences, 2008
Other uses of charcoal records:An index of “biomass burning”
Composite index = mean of z-scores of box-cox transformed charcoal accumulation rates
Maximum sampling resolution among sites = 20 year (reduces influence of high-resolution sites)
Full-day workshop at the 2010 Annual Meeting of the Ecological Society of America, Pittsburgh, PA.
July 31, 2010
Developing and interpreting fire histories from sediment charcoal records
Lead organizers: Daniel Gavin and Phil Higuera
Summary sentence: Recent developments in the reconstruction of fire histories from sediment charcoal data have opened the door to making landscape-scale interpretations of disturbance regimes over millennia. In this workshop we will discuss methods, analytical tools, and ecological interpretation of charcoal-based paleofire records.